The Pope in Winter: The Dark Face of John Paul II's Papacy by John Cornwell 329pp, Viking, £20

There is a story that a visitor at an audience with Pope John Paul II in the Vatican was shocked by the elderly pontiff's Parkinson's-racked appearance. He bent low over the shaking and sagging figure and asked him how he was. A beady eye glimmered out of the pink face sunk against the papal chest and the slurred voice crackled out: "From the neck down, not so good." Then it added, fiercely: "But I don't lead the church with my feet!"

His Holiness may be a bit beyond that sort of sally now but, true or not, the story of the pope's indomitability is entirely in character and it is well-illustrated by the dustjacket of this book. The cover, wrapped around a critical biography by a well-known Vatican watcher, depicts John Paul II, buffeted and faltering, leaning King Lear-like for support against his crucifix in what appears to be a strong gale. It is a picture of human frailty.

The ailing Pope has published two volumes of reflections in recent months, one last autumn and another coming out here next month. His publishers must (understandably, for they are only human) be hugging themselves at the serendipity of this timing, though it may be doubted how much John Paul II has really had to do with writing the books in his declining condition. His bland thoughts will probably receive a large sale, especially if he dies, though they have apparently not sold well yet. But perhaps The Pope in Winter deserves the wider study. John Cornwell has produced a devastating report. Catholics should read it, if not to change their views - though perhaps it should - then at least to inform them.

Cornwell wrote the book that skewered the reputation of the wartime Pope, Pius XII, for vacillation and cowardice in the face of the Nazis - an exposé for which some Catholic loyalists still cannot forgive him and which caused an American nun to try to throttle him on live TV. And now he sets fair to saw John Paul II off at the knees as well. The loyalists and hagiographers are circling defensively around the old boy to protect him from the attack on his record, though they have been unable to undermine any charges of substance. John Paul II's supporters believe the Pope is almost devoid of human stain. Sometimes the pontiff, after a quarter of a century of infallibility, seems to think so too. This book shows that, like the rest of us, he is all too human.

John Paul II, the Polish pope, born Karol Wojtyla 84 years ago in Wadowice, is perhaps the most extraordinary and influential Christian of modern times. He has been a pope like no other: not just because of his longevity; not just for supplanting the centuries-old tradition that popes must be Italian; not just for rising triumphantly above an obscure and oppressed background in Nazi-occupied Poland and then communist eastern Europe to lead the largest Christian denomination in the world for nearly three decades; but because he grasped and shaped a remote and increasingly ineffectual office and gave it real influence and respect in the secular world. The papacy will never be the same again. Never again will the college of cardinals - almost all of whom have now been appointed by John Paul II - be able to elect a decrepit figure who can safely be immured, isolated and lonely, within the walls of the Vatican until blessedly released by death.

In these days when the Pope is obviously declining, his imminent death forecast for more than a decade, often by men who are now themselves dead, it is sometimes hard to recollect just how dynamic John Paul II was at the start of his papacy. For two decades he was a sturdy, evidently holy figure, firm in his faith, a formidable theologian but a still more redoubtable politician, dynamic in his actions, shuttling around the world, sinking to his knees to kiss the tarmac of every airport on which he landed, stentoriously preaching to fervent audiences running into the hundreds of thousands. In 1981 an assassination attempt by a Turkish gunman, probably hired by Bulgaria's communist government, came closer to killing him than the Vatican liked to admit. Throughout the 1980s the Pope's moral rectitude was pivotal in challenging and undermining the Polish regime and, through it, the edifice of the eastern bloc. No wonder Brezhnev thought it a mistake to allow him back into Poland for a visit. Unsurprising if some thought it would be better if he was dead.

The Pope's dynamism and charisma were huge, rejuvenating strengths for the world's 1bn Roman Catholics, but they contained weaknesses which, for many of us faltering followers, have undermined our faith in him. Cornwell insists that they have produced a dogmatic certainty and authoritarianism that has attempted to cement together a diverse worldwide communion in a single, reactionary framework and which has taken little account of local circumstances or traditions of episcopal autonomy.

It is in the nature of popes to be authoritarian - we're not talking about the consensual Church of England here - but this Pope has been particularly domineering and intolerant, not just of dissent but also of any opinions other than his own. His background was, after all, not in a democracy. More damaging for the institution, as his health has faltered, so has the grip of the small coterie that surrounds him tightened - and they are an intransigent, introspective, reactionary and uninspirational bunch. Hence entirely unnecessary rows with other denominations over whether they can properly be regarded as Christian and clodhopping injunctions on whether gays are evil or girls can sing in church choirs.

Much more seriously, this has forced the church into damaging and destructive positions, protecting paedophile priests and insisting that condoms do not protect against Aids. Meanwhile the Pope has until very recently still been busily creating cardinals and saints and shuttling round the world, invoking the awe and devotion of his adoring fans, even if he can no longer fluently or coherently address them. He is locked on to a rigid treadmill of duty that can end only with his death, as the statement issued on his behalf from his bed at the Gemelli hospital last weekend made clear.

He is convinced of his divine mission, secure in the protection of the Virgin Mary, persuaded that her intervention - as foretold to the Portuguese peasant children of Fatima three years before his birth - deflected the assassin's bullet from his vital organs. Cornwell quotes a Polish friend: "People around him see the sweetest, most modest person [but] he is by no means as humble as he appears. Neither is he modest. He thinks about himself very highly, very adequately."

How is the church going to follow such an act? It cannot go back to electing octogenarians in a world of dynamic, youthful leaders. But can it bear another such as John Paul II?